Add keyboard shortcut for default prompt

[jeba57]

Summary

Implements Option B from #917 — adds a dedicated keyboard shortcut to open the Side Panel and automatically submit the user's configured default prompt.

Fixes #917

1. Ctrl+Shift+S opens the Side Panel and automatically submits the configured default prompt.
2. Added the execute_predefined_prompt command in wxt.config.ts.
3. Added background handling in background.ts to open the Side Panel and trigger prompt submission.
4. Updated sidepanel-chat.tsx to resolve and submit the saved default prompt automatically.
5. Added validation for missing prompts, system prompts, empty prompts, and lookup failures.
6. Existing Ctrl+Shift+Y Side Panel shortcut and existing behavior remain unchanged.
7. Manually tested successfully — the Side Panel opened and the default prompt was submitted automatically without clicking Send.

Testing

• Configured a default prompt under Settings → Default Prompt for SidePanel (Copilot).
• Pressed Ctrl+Shift+S with the Side Panel closed.
• Verified the Side Panel opened and submitted the prompt automatically.
• Verified the AI response was generated successfully.

Demo

Side Panel after pressing Ctrl+Shift+S — default prompt auto-submitted and response received.